Controllers for resistance type welding machines are described, for example, in copending application Ser. No. 361,820 filed Mar. 25, 1982 in the name of the present inventor, now U.S. Pat. No. 4,456,809 and in U.S. Pat. No. 4,301,351 which issued in the name of Mathews, both of which are assigned to the present assignee. Both these controllers include circuits for compensating for the effect of line voltage variations on tip temperatures, this being achieved by digitizing the RMS value of the input voltage and by computing the proper firing time during each half-cycle of input voltage and by utilizing the results to maintain constant power to the welding tips in the presence of line voltage fluctuations.
The circuit and system of the present invention controls the effective RMS primary or secondary current, rather than input voltage, to eliminate the effects of load changes, as well as line voltage fluctuations, and thereby maintain constant tip temperatures in the presence of both line voltage fluctuations and load changes.